Evolution
How life changes over generations through variation, heredity, and natural selection: the single biggest idea in biology.
Descent with modification
The central idea that all living things share common ancestry and change gradually across generations.
Variation and heredity
Why individuals differ and how those differences pass to offspring: the raw material evolution works on.
Natural selection
Darwin's mechanism: when heritable variation affects survival and reproduction, populations change over generations.
Adaptation
How selection shapes the fit between organism and environment, and the limits and imperfections that reveal how it works.
The origin of species
How a single lineage splits into two through isolation and divergence: the branching that fills the tree of life.
The evidence
Fossils, shared anatomy, DNA, and evolution observed in real time: why common descent is so firmly established.